Optical fiber has taken off from its embroyonic stage as a communication medium about a dozen years ago to its present status as a world class medium for reliable communications. Typically, optical fiber includes a fused silica portion comprising a core and a cladding and one or more layers of coating material. The coating material protects the fiber which is drawn from a preform.
It has been conventional that the optical fiber coating materials be acrylate-based ultraviolet curable materials which include photoinitiators. These materials cure, i.e., are converted from a liquid to a solid, by what is referred to as free radical cure. In a free radical cure system, upon absorption of light, the photoinitiator component cleaves to form a pair of free radicals. This free radical pair diffuses first from each other and reacts with arcylate-terminated components to initiate a free radical chain polymerization process. Acrylate-based coating systems are known to be chemically neutral, that is neither acidic or basic by design.
Another group of coating materials are cationically curable. In cationic cure, a cation or a proton is both the initiating and the propagating species in the polymerization mechanism. See U.S. Pat. No. 4,956,198 and PCT application publication No. WO90/03988. In the latter, cationic cure, which may be used on fiber coatings is such that polymerization continues even after the exposure to curin energy has been discontinued, whereas in free radical cure, the polymerization reaction may be incomplete inasmuch as the reaction discontinues when exposure to curing energy is discontinued.
It has long been desired to increase the strength of the optical fiber from its present prooftest value of about 50,000 psi and to cause the strength to be uniform throughout a length of fiber. An optical fiber which has been provided with a hermetic coating has a prooftest value which may reach 200,000 psi. However, difficulties experienced in the manufacture and coloring of hermetic coated optical fiber have hindered its widespread acceptance.
It is known that fused silica optical fiber strength is higher after aging in acidic environments than it is after aging in neutral or passive environments. See, for example, H. C. Chandan and D. Kalish "Strength and Dynamic Fatigue of Optical Fibers Aged In Various pH Solutions" which appeared at pp. 12-14 of the Proceedings of the Topical Meeting on Optical Fiber Communication, Washington, D.C. 1979.
With the quest for uniform strengths, there is a desire to provide a mechanism for causing fiber strengths to be higher and more uniform. A long-felt need for optical fibers has been to increase their strength and resistance to stress cracking. Such properties would result in a more reliable communication system. The resultant optical fiber could be used in applications where higher strengths are required, such as in underwater cable or in tethered vehicles. Seemingly, the art has not yet provided a solution to the problem of increased fiber strengths which solution would not detract from cure speed and which solution would be relatively easy to implement.